This proposal is based on recent findings from the applicant's lab indicating that the PDE inhibitory subunit, gamma, plays important regulatory roles in phototransduction that had not been previously recognized. In particular, the applicant and his colleagues have shown that the PDE- gamma subunit is phosphorylated and ADP-ribosylated. Experiments are proposed that will further define the roles of phosphorylation and ADP- ribosylation. The kinase and ADP-ribosyltransferase that modify PDE- gamma are to be purified and characterized. Additional experiments are proposed to investigate the mechanism(s) that regulate the rate of hydrolysis of GTP by transducin.